Hi Lin Lin,

First of all, we appreciate very much that you have taken the time to read and ask about our study! We will try and address your questions listed below as best as possible here, but definitely also make sure to clarify these aspects when revising the article - so thank you very much for helping us improve it. 

Question 1: If I understand your methodology correctly, your observed INP number as a function of temperature is in the immersion freezing mode. However, Meyers scheme (M92) describes the deposition freezing and condensation ice nucleation. Could you please explain a bit more why the measured INP concentrations that are relevant for the immersion mode can be used to replace the M92 scheme?

Regarding your first question, this is a slightly unfortunate complication due to CAM5’s parameterization choices, which is an outdated treatment of ice nucleation. It is true that the Meyers’ scheme (M92) describes deposition nucleation and condensation-freezing in mixed-phase clouds, and it is also the dominant ice nucleation pathway in CAM5. We replace this parameterization with a new function (A21) based on measurements of INPs in immersion freezing mode, which is now being accepted as the dominant nucleation pathway in mixed-phase clouds (see e.g. Ansmann et al., 2009, de Boer et al., 2011, Westbrook and Illingworth, 2011). 

We clarify a few additional details of our ice nucleation modifications in CAM5. It should be stressed that we do not make any changes to the deposition nucleation scheme in cirrus clouds, but only change the M92 scheme active for temperatures between -37°C and 0°C . Additionally, in the updated version of this manuscript we exclude the immersion freezing parameterization of Bigg wherever we replace M92, so that the A21 parameterization is the only active immersion freezing parameterisation in the Arctic. This makes very little change to the cloud ice number concentrations or the overall picture we see, as the Bigg parameterization contributes little to activated ice nuclei, something which is also stated by previous studies by e.g. English et al. (2014). To address the unclarities you have pointed out, we plan to update lines 148-156 accordingly:

“In CAM5, the different heterogeneous ice nucleation pathways in mixed-phase clouds are parameterized independently, namely, contact freezing (Young, 1974), immersion freezing (Bigg, 1953, hereafter: "B53") and deposition and condensation freezing (Meyers et al., 1992, hereafter: "M92"). Here, we update the M92 parameterization. This parameterization is active in the temperature range -37°C to 0°C, and is responsible for more than 90 % of ice nuclei formed in CAM5 mixed-phase clouds (English et al., 2014). Since the measured INP concentrations are relevant for the immersion mode, replacing the M92 with our measurements entails excluding deposition and condensation freezing in Arctic mixed-phase clouds. This exclusion is justified by observational studies that found deposition and condensation freezing to be negligible for mixed-phase clouds (Ansmann et al., 2009; Boer et al., 2011; Westbrook and Illingworth, 2011). Where we update the M92 parameterization using our INP measurements in immersion freezing mode, we exclude the B53 immersion freezing parameterization.”

Question 2: Is it appropriate to quantify INP concentration as a function of temperature in the immersion freezing mode? For visualization purpose, INP number concentration can be plotted as a function of temperature, like figure 2 in your paper, figure 7 in Geerts et al., (2022) (https://journals.ametsoc.org/view/journals/bams/103/5/BAMS-D-21-0044.1.xml). But can we quantify INP number concentrations as a function of only temperature in the immersion freezing mode, without considering other factors such as water activity or active surface area density?

Regarding your second question, it is true that we make a simplification when we predict immersion freezing INP concentration based only on temperature. The reason we do not include aerosol surface area density as a predictor in the parameterisation is because we find quite low correlation with aerosol surface area density (see Fig. A1e in the appendix). This finding is consistent with INP measurements in other Arctic sites, e.g. in Ny-Ålesund (Li et al., 2022). Therefore we believe that including aerosol surface area density will not improve the predictive ability of our INP parametrization. The aerosol species originally used to predict INPs in CAM6-Nor (the atmospheric model we use) are dust and soot, but other sources such as marine organic aerosols or other bioaerosols are likely important Arctic INPs (see for example Creamean et al. (2022), Carlsen and David (2022), Sze et al. (2023) or Freitas et al. (2023)). Differences in water activity can indeed result from different aerosol chemical composition and in theory affect ice nucleation. However, this is difficult to account for with our measurement technique, as the aerosols are all impinged in water before their ice-nucleating ability is probed, which may dissolve and dilute substances that would otherwise affect the freezing ability of an aerosol. However, one of our motivations for using a simple temperature-based parameterisation is that we do not yet have a full understanding of how INP concentrations can be predicted in the Arctic. Therefore by keeping it simple, we avoid introducing any additional sources of error when predicting Arctic INPs, until a full understanding on the composition-dependent ice-nucleating ability of Arctic INPs is in place.

Question 3: How different is your measured INP number concentration from that measured during the COMBLE campaign? One year apart in the same season yield similar results?

Regarding your third question, if we compare Fig. 7 in Geerts et al. (2022, https://doi.org/10.1175/BAMS-D-21-0044.1) and Fig. 2 in our study we see that our 2021 measurements in Andenes one year after the COMBLE campaign show somewhat higher maximum concentrations (around one order of magnitude at some temperatures) than the COMBLE campaign, and similar minimum concentrations. It should be noted that there are certain important differences between the data. First of all, the COMBLE data exclusively represents cold air outbreaks, i.e. flow reaching Andenes from the Arctic, while our measurements also include air masses reaching our measurement site with a more southerly and southwesterly flow (see Fig. A2 of the air mass back trajectories in the appendix). As these air masses are going into the Arctic, we believe that they are also important to capture in our case to understand Arctic INP concentrations. Secondly, our measurements are only from March 2021, while the COMBLE measurements are from the period December 2019 to May 2020. Nevertheless, we find that our measurements are quite similar to other measurements around the Arctic (see Fig. 2). Additionally, our measurement technique does not allow us to estimate INP concentrations higher than approximately 0.1/L, or INP concentrations at temperatures lower than around -25°C. This should be kept in mind when comparing the two datasets. To make this comparison easier for the reader, we are in the process of incorporating the COMBLE data points into Fig. 2 in the manuscript.

Thank you again for your questions and  please let us know if you have any others.

Best regards,

Astrid Bragstad Gjelsvik on behalf of the author team

Papers on liquid as a prerequisite for ice in mixed-phase clouds:

Ansmann, A., M. Tesche, P. Seifert, D. Althausen, R. Engelmann, J. Fruntke, U. Wandinger, I. Mattis, and D. Müller (2009), Evolution of the ice phase in tropical altocumulus: SAMUM lidar observations over Cape Verde, J. Geophys. Res., 114, D17208, doi:10.1029/2008JD011659. 

de Boer, G., H. Morrison, M. D. Shupe, and R. Hildner (2011), Evidence of liquid dependent ice nucleation in high-latitude stratiform clouds from surface remote sensors, Geophys. Res. Lett., 38, L01803, doi:10.1029/2010GL046016. 

Westbrook, C. D., and A. J. Illingworth (2011), Evidence that ice forms primarily in supercooled liquid clouds at temperatures > −27°C, Geophys. Res. Lett., 38, L14808, doi:10.1029/2011GL048021. 

Paper from English et al. on ice nucleation parameterizations in CAM5:

English, J. M., Kay, J. E., Gettelman, A., Liu, X., Wang, Y., Zhang, Y., & Chepfer, H. (2014). Contributions of Clouds, Surface Albedos, and Mixed-Phase Ice Nucleation Schemes to Arctic Radiation Biases in CAM5. Journal of Climate, 27(13), 5174-5197. https://doi.org/10.1175/JCLI-D-13-00608.1

Paper on INP measurements in Ny-Ålesund:

Li, G., Wieder, J., Pasquier, J. T., Henneberger, J., and Kanji, Z. A.: Predicting atmospheric background number concentration of ice-nucleating particles in the Arctic, Atmos. Chem. Phys., 22, 14441–14454, https://doi.org/10.5194/acp-22-14441-2022, 2022. 

Papers on INPs, seasonality and connections to bioaerosols:

Creamean, J.M., Barry, K., Hill, T.C.J. et al. Annual cycle observations of aerosols capable of ice formation in central Arctic clouds. Nat Commun 13, 3537 (2022). https://doi.org/10.1038/s41467-022-31182-x

Carlsen, T., & David, R. O. (2022). Spaceborne evidence that ice-nucleating particles influence high-latitude cloud phase. Geophysical Research Letters, 49, e2022GL098041. https://doi.org/10.1029/2022GL098041

Sze, K. C. H., Wex, H., Hartmann, M., Skov, H., Massling, A., Villanueva, D., and Stratmann, F.: Ice-nucleating particles in northern Greenland: annual cycles, biological contribution and parameterizations, Atmos. Chem. Phys., 23, 4741–4761, https://doi.org/10.5194/acp-23-4741-2023, 2023. 

Pereira Freitas, G., Adachi, K., Conen, F., Heslin-Rees, D., Krejci, R., Tobo, Y., Yttri, K. E., & Zieger, P. (2023). Regionally sourced bioaerosols drive high-temperature ice nucleating particles in the Arctic. Nature communications, 14(1), 5997. https://doi.org/10.1038/s41467-023-41696-7